Mesoporous materials are nothing but porous materials, having uniform pores of 2 to 50 nm in diameter. Mesoporous polymeric architecture has received great attention in recent days owing to its effectiveness in water purification and gas separation. There are various methods available to develop durable mesoporous polymeric materials. Industries generally adopt solvent or heat energy to induce phase separation for producing porous polymer membranes economically.
Freeze drying of polymer solutions to achieve the controlled smaller porous structure has also been constantly explored by academicians and industries. In one way, electrospinning technique can also be considered in this category as the process freezes the polymer in the form of nanofibers after evaporating solvent, leaving as fine narrow porous fiber network. However, electrospinning is not successful in producing smaller mesopores structure for all the industry common polymers, and the surface area is still not great enough. Chemical (porous inorganics) templates were explored to produce smaller mesopores. Nevertheless, the major difficulty in general is, controlling the diffusion of solvent molecules. So it remained very challenging to fabricate mesoporous materials through the phase separation of common polymers.
The scientific team from National Institute for Materials Science-Japan, Japan Science and Technology Agency, Core Research for Evolutional Science and Technology, East China University of Science and Technology, Zhejiang Universityand led by Dr. Izumi Ichinose introduced flash freezing route to generate mesoporous nanofiber networks with high specific surface area. The team restricted the solvent diffusion to the nanometre range by operating under deep frozen conditions, i.e. by crystallizing the solvent molecules far below freezing point temperature which resulted in extremely fine microphase separation, and upon extracting the solvent molecules at low temperature, an interconnected polymer nanofiber network were formed. Their fabrication method of mesoporous polymer actually included 3 key steps as follows:
A concentrated polymer solution was first frozen using liquid nitrogen. This process facilitated the solvent molecules subsequently transit to a glassy state between the polymer chains. Rapid freezing in liquid nitrogen generated transparent frozen solutions, indicating that macrocrystals were not formed. Flash freezing modified the solution without allowing solvent crystallization.
In the second step, the temperature was slowly increased to produce cold crystallization of the frozen solvent molecules. During this crystallization, the polymer chains were expelled from solvent nanocrystals, producing a nanofiber network structure. This crystallization process resulted in an increase of the polymer concentration within the polymer-rich phase.
Finally, extracting the nanocrystallized solvent at low temperature yielded a highly porous nanofiber network.
|Mesoporous material can be compared to honeycomb - only the scale is significantly different…|
Industrial important polymers PES and PSF were demonstrated as mesoporous network using this method, which exhibited interesting separation properties, thanks to high surface area and percolating pores. The resulting polymer nanofiber networks obtained from 40 wt% PSF solutions exhibited pore radii of 2.7 nm. The polymer nanofiber networks showed large surface area over 300m2/g.
These mesoporous nanofiber networks were able to rapidly adsorb and desorb a large amount of CO2, about three times larger than that of the bulk polymer. Mesoporous PES was capable of separating toluene from aqueous solutions a few tens of ppm in concentration. It was also effective in adsorbing a significant amount of tetrahydrofuran (a corrosive solvent) from aqueous solution.
The adsorption results were found more than two times greater than the best values obtained for commercially available activated carbons and cross-linked polymer adsorbents in comparison experiments.
The team is confident that mesoporous polymer membranes developed by this method will contribute to the purification of industrial oil and gas fields. It is expected that mesoporous PSF sheets in ultrafiltration process could achieve over 90% rejection efficiency for 5 nm particles.
This excellent work has been published in prestigious ‘Nature Communication’. It can be accessed at this link;http://www.nature.com/ncomms/2013/131022/ncomms3653/full/ncomms3653.html